1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor containing asymmetric naphthalenetetracarboxylic acid diimide derivatives and an electrophotographic imaging apparatus employing the derivatives. More particularly, the invention relates to an electrophotographic photoreceptor containing naphthalenetetracarboxylic acid diimide derivatives having good solubility in an organic solvent and high compatibility with a polymeric binder resin. The invention is also directed to an electrophotographic imaging apparatus employing the electrophotographic photoreceptor.
2. Description of the Related Art
An electrophotographic photoreceptor is used in electrophotography applied to laser printers, photocopiers, CRT printers, facsimile machines, LED printers, liquid crystal printers, and laser electrophotos, and the like. The electrophotographic photoreceptor comprises a photosensitive layer formed on an electrically conductive substrate. The substrate can be in the form of a plate, a disk, a sheet, a belt, or a drum, etc. In electrophotography, an image is formed using an electrophotographic photoreceptor. First, a surface of the photosensitive layer is electrostatically charged uniformly, and then the charged surface is exposed to a pattern of light, thus forming an image. The light exposure selectively dissipates the charge in the exposed regions where the light strikes the surface, thereby forming a pattern of charged and uncharged regions, which is referred to as a latent image. Then, a wet or dry toner is applied in the vicinity of the latent image, and toner droplets or particles deposit in either the charged or uncharged regions to form a toner image on the surface of the photosensitive layer. The resulting toner image can be transferred and fixed to a suitable ultimate or intermediate receiving surface, such as paper. Alternatively, the photosensitive layer can function as the ultimate receptor for receiving the image. The residual toner then is cleaned from the electrophotographic receptor and residual charges are erased. Thus, the electrophotographic photoreceptor can be used repeatedly for long periods.
Electrophotographic photoreceptors are generally categorized into two types. The first is a laminated type having a laminated structure including a charge generating layer comprising a binder resin and a charge generating material (CGM), and a charge transporting layer comprising a binder resin and a hole transporting material (HTM). In general, the laminated type electrophotographic photoreceptor is used in the fabrication of a negative (−) type electrophotographic photoreceptor. The other type is a single layered type in which a binder resin, a CGM, an HTM, and an electron transporting material (ETM) are contained in a single layer. In general, the single layered type photoreceptor is used in the fabrication of a positive (+) type electrophotographic photoreceptor.
The (+) type single layered electrophotographic photoreceptor is advantageous in that it generates a small amount of ozone harmful to humans and since it has a single photosensitive layer, its production costs are low. The most essential material among the materials composing the (+) type single layered electrophotographic photoreceptor is the ETM. Since the hole transporting ability of the HTM is at least a hundred times greater than the electron transporting ability of the commonly used ETM, the performance of the single layered electrophotographic photoreceptor is dependent upon the electron transporting ability of the ETM.
The electron transporting ability of the ETM is greatly affected by its solubility in an organic solvent and compatibility with a polymer binder resin. The conventional ETM includes, for example, a dicyanofluorenone derivative having Formula (i) below, a diphenoquinone derivative having Formula (ii) below, a naphthalenetetracarboxylic acid diimide derivative having Formula (iii) (see U.S. Pat. Nos. 4,992,349 and 4,442,193), and an o-substituted naphthalenetetracarboxylic acid diimide derivative having Formula iv (see U.S. Pat. No. 6,127,076).

wherein
R1 is a substituted or unsubstituted alkyl group or an aryl group, etc.

wherein
R1, R2, R3, and R4 are independently a substituted or unsubstituted alkyl group, a cycloalkyl group, an aryl group, or an alkoxy group,

wherein
R1 and R2 are independently a substituted or unsubstituted aryl group, a sulfonyl group, a sulfone group,
R3, R4, R5, and R6 are independently a hydrogen atom, a C1-4 alkyl group, a C1-4 alkoxy group, or a halogen atom, and
n is 0-3,

wherein
R is a hydrogen atom, an alkyl group, an alkoxyl group, or a halogen atom,
R1 and R2 are different from each other and each is a substituted or unsubstituted alkyl group, an alkoxyl group, or an aryl group,
R3 is a hydrogen atom, a substituted or unsubstituted alkyl group, an alkoxyl group, or an aryl group.
The dicyanofluorenone derivative of Formula (i) and the diphenoquinone derivative of Formula (ii) have low solubility in an organic solvent and low inherent electron transporting ability. Thus, electrophotographic photoreceptors manufactured using the derivative (i) or (ii) as the ETM have disadvantages such as a remarkably reduced charge potential and an increased exposure potential after repeated charging exposures.
The naphthalenetetracarboxylic acid diimide derivatives of Formulae (iii) and (iv) are known to have high electron transporting ability. However, these derivatives of Formulae (iii) and (iv) have low solubility in an organic solvent and low compatibility with a polymer binder resin. Electrophotographic photoreceptors manufactured using these derivatives have surfaces of the photosensitive layers that may crystallize (precipitation of ETM), thus adversely affecting the electrostatic properties of the photoreceptors.
Thus, electrophotographic photoreceptors, especially single layered type electrophotographic photoreceptors, manufactured using the conventional ETMs have a remarkably reduced charge potential and an increased exposure potential after repeated use. In general, surface charges of electrophotographic photoreceptors must be maintained at a predetermined potential. Due to the decrease in the charge potential and the increases in the exposure potential, image qualities may be deteriorated.